1. Field of the Invention
The present invention relates to a conveying apparatus, an image forming apparatus including the conveying apparatus, and a conveying method.
2. Description of the Related Art
For example, paper feed precision in inkjet printing is known to greatly affect a printing quality. In particular, when the head width and the conveyance amount are the same and when feed precision in joints of raster scanning is bad, stripes or voids are generated in an image, which severely degrade the image quality.
Incidentally, in the inkjet printing, in order to improve an image quality, resolution is increased and the number of dots per unit area is increased. However, there is a limitation in decreasing the pitches of ink ejecting nozzles, because of a processing problem, and about 300 [dpi] (=84.7 [μm]) becomes the limitation at the present time.
For this reason, in order to realize high resolution, it is considered to convey paper to cause ejection ports of the nozzles to come between dots, which are already struck on the paper at the pitches of 84.7 [μm], and strike the dots. In actuality, a sheet carrying technology is already known for mounting a high-resolution encoder on a paper carriage roller shaft and conveying a paper between the dots with high precision while performing feedback control.
Since the resolution of the current encoder is limited to about 2 to 4 [μm] and an excessively large cost is required to achieve an encoder with lower resolution, and the encoder with the lower resolution is not practical for a consumer product. The resolution of [10 μm] or more results in adverse effects of stripes to the sight. For this reason, when the head width and the conveyance amount are the same and when the paper can be conveyed at precision of 2 to 4 [μm] corresponding to the encoder resolution, a feed error does not becomes an error of level that causes a problem on an image.
When ink is repetitively ejected between the dots already struck to realize high resolution, dots may strike to be overlapped to the same dots to realize high density. In this case, circumstances become different. That is, when the paper is repetitively conveyed at precision of 2 to 4 [μm] (for example, 3 [μm]) corresponding to the encoder resolution, an error of 3 [μm] (±1.5 [μm] to be exact) is generated by a feed count.
Accordingly, when the dots are repetitively struck 16 times, a maximum deviation of 3×16=48 [μm] is generated. In the final feed, when the resolution is 600 [dpi], the deviation of about one dot or more is generated.
If the such a deviation is generated, smoothness of the image is deteriorated (deterioration of granularity), interference occurs between the deviated dots, and density irregularities of moire stripes are generated.
A method that prevents an image quality from being deteriorated due to the stripes included in the image is described in Japanese Patent No. 3,121,432. If the conveyance amount of recording paper is deviated from an encoder value due to sliding between the roller and the paper or thermal expansion of the roller, positional deviation in joints in one raster is generated, and an image quality is deteriorated due to the stripes included in the image. For this reason, in order to prevent the image quality from being deteriorated, in the above method, used are a first encoder to output a pulse signal corresponding to the rotation speed of a motor and a second encoder to output a pulse signal corresponding to the conveyance speed of the recording paper. Furthermore, in the above method, the actual conveyance amount of the recording paper on the assumption of the resolution obtained from the first encoder and a minimum raster number are calculated as a correction coefficient. Thereby, a deviation with the target conveyance amount is absorbed.
In the method that is disclosed in Japanese Patent No. 3,121,432, as described above, the two encoders are needed, and correction of ±1 pulse is performed when a count reaches a predetermined count. For this reason, an error is removed when the correction is performed while the error is gradually accumulated until the correction is performed. As a result, periodic irregularities are generated in the conveyance amount.